Creating the Digital Image Flashcards
What are all digital images composed of?
A matrix of digital values
What is a matrix?
Columns and rows that forms boxes that represent the pixels in an image
How are the values of a matrix organized (laid-out)?
In columns and rows
If there are the same number of rows and columns what shape are the boxes (pixels)?
Square
If there are different number of rows and columns what shape are the boxes (pixels)?
Asymmetric which is used in MRI
Where are brightness values stored in a computer?
In the memory, which each pixel value having a particular brightness level
What is each cell of a picture called?
Picture element aka Pixel
What is the general size of pixels?
Sub-millimeter around 0.7 mm
What do pixels directly affect in an image?
The spatial resolutionW
What is the relationship between pixels and spatial resolution?
The more pixels you have, the smaller they are and the better the spatial resolution is in the image
What type of matrix will produce the best resolution images?
Large matrix since they have more pixels
How do you calculate the actual total number of pixels?
Multiply the columns by the rows
What is the relationship between matrix size and pixel size?
As the matrix size increases the pixel size decreases
What is the relationship between matrix size and pixel quantity?
As the matrix size increase the number of pixels increases
What is spatial resolution measured in terms of?
Measured in terms of spatial frequency in LP/mm (line-pairs per millimeter)
To record a line pair, what is the minimum number of pixels needed?
At least two, one bright and one dark
What is the resolution of modern digital systems?
6-8 LP/mm
What is the formula to calculate pixel size?
Pixel size = FOV/matrix
What is a field of view (FOV)?
Border of the matrix, given in inches, cm or mm but will need to be converted to mm for the purpose of the pixel size equation
What is the pixel size for a 12 x 12 digital image reconstructed at a display screen on a 1024 x 1024 matrix?
- Convert the field of view from inches to mm
1”=25.4 mm
12” x 25.4 = 305 mm - Complete the pixel size formula
PS=FOV/matrix
PS=305/1024
PS=0.298 rounded up to 0.3 mm
What is the formula to calculate spatial frequency in terms of line pairs?
SF= 1/2(PS)
What is the spatial frequency in terms of line pairs of a pixel that is 0.3 mm?
SF= 1/2(.3)
SF=1/.6
SF= 1.67 LP/mm
What is attenuation coefficient?
The percentage of the original x-ray beam intensity that is absorbed by the tissue
How does the attenuation coefficient relate to brightness in the image?
As the attenuation coefficient increases, the brightness increases and the image will have a lighter grey appearance
What is the attenuation coefficient of bones?
High attenuation coefficient since more radiation is absorbed by the bone and less makes it to the IR
How do bones show in radiographs in terms of brightness?
Bones will appear brighter becuase of their higher attenuation coefficient
What is the attenuation coefficient of soft tissues?
Low attenuation coefficient since less radiation is absorbed by the tissue and more makes it to the IR
How do soft tissues show in radiographs in terms of brightness?
Appear darker since the attenuation coefficient is lower
What does voxel stand for?
Volume element
What are voxels?
Three dimensional cubes containing tissue information of different types
How do the different tissue types in a voxel affect the attenuation coefficient?
The attenuation coefficient is based on the average amount of attuation across all tissue types within the voxel, which then determines the image brightness of the pixel
If a voxel only contains bone as the tissue type, how will the average attenuation coefficient be affected?
Average attenuation coefficient will be higher since there are not many differences in the attuation by tissue type (Image will be bright)
If a voxel contains bone and tissue as the tissue type, how will the average attenuation coefficient be affected?
It will be lower, since its taking into account the attenuation differences in bone and tissue (which vary greatyl) which will make the image not as bright
What is pixel bit depth?
The maximum number of brightness values of a given pixel
How else can pixel bit depth be expressed?
How many different shades of greay can any one pixel display
What is the formula to calculate bit depth?
Bit depth = 2^x
If you have a bit depth of 6, how many different shades of gray will that pixel be?
Bit depth = 2^x
Bit depth = 2^6
Bit depth = 64 shades of grey
How many shades of grey can the human eye discern?
32 shades of grey on average
What is dynamic range?
The maximum number of brightness values of an entire system (computer hardware and software)
How is dynamic range measured?
By calculating the bit dept of all pixels
What is the dynamic range of most digital systems?
2^8 (256), 2^10(1024) or 2^12 (4096) gray level
What does a higher dynamic range indicate?
The higher the dynamic range, the more gray scales in the image (long scale contrast).
If a system has a higher dynamic range, what does that mean for image details?
The higher dynamic range allows for more details and smaller details to be seen in an image
How is the file size of an image calculated?
Matrix size x bit depth
What is the relationship between file size, matrix size and bit depth?
If matrix size or bit depth increase, the file size will also need to increase
What benefit does digital processing have on original attenuation coefficients?
It allows you to manipulate the attenuation coefficients and change contrast resolution which wasn’t available in film
What level of differences does digital processing allow?
Allows the system to distinguish contrast resolution differences as low as 1%
What is an example of a digital system processing change?
Lightening greyscale of some pizels to highlight differences between objects
What is a procedural algorithm?
A preset greyscale and gray level that are selected by procedure
Once a procedural algorithm is selected what changes are made to the image?
The algorithm will adjust the brightness and contrast in your image to match a predetermined system level of contrast/brightness that most people find acceptable
What is windowing?
A post-processing technique that allows you to change the grey scale (contrast) of the image after its displayed and gone through pre-processing
What does window width control?
The contrast (long/short scale) in an image
What type of contrast is associated with a wide window width?
Long scale/low contrast
What type of contrast is associated with a narrow window width?
Short scale/high contrast
What does window leveling control?
The brightness/density of the image
What window width and levels should be used to demonstrate soft tissue structures in a digital image?
Wide width = long contrast
Low window level = low brightness
What effect does a higher window level have on an image?
Increasing window levels make the image on the monitor darker
What should the optimal window level be kept on a number scale?
0
- anything below 0 will be brighter
- anythin above 0 will be darker
What do digital workstations allow?
Manipulation of images and storage to PACS
What is the typical monitor setup for radiologists?
Two large monitors side by side that are 14 x 17
